1. Field of the Invention
The invention relates to a method for applying a layer of fibers coated with a tacky inorganic binder onto a surface. More particularly, the invention is a method for spraying a layer of refractory fibers coated with a tacky binder including a montmorillonite clay. By way of further characterization, but not by way of limitation thereto, the invention is a method for spraying refractory fibers which have been coated with the chromium aluminum phosphate binder including a bentonite clay onto a surface and curing the coated fibers.
2. Description of the Prior Art
In the past, high temperature resistant fibrous material has been applied to heated surfaces, such as heat treating furnaces and kilns, by attaching batts or strips containing such fibers to the heated surface. This method requires mechanical anchoring or fastening means to attach the strips to the heated surface. Such mechanical attachment is expensive in that it requires large amounts of labor and fastening materials to attach the strips. In addition, because this method is time consuming, it requires the furnace or kiln to be out of service for extended periods of time. Finally, gaps between adjoining strips may occur either during the attachment process or later due to thermal shrinkage which allows heat leakage between the layers of fiber.
Improvements to this method of attaching refractory fiber linings to the wall of kilns and furnaces have been made such as is described in U.S. Pat. No. 4,381,634. This method involving the mechanical attachment of modules, overcomes many of the problems associated with layered linings especially the opening of gaps between the edges of layers due to heat shrinkage at elevated temperatures. However, like the above method, the use of modules requires mechanical attachment to the metal shells of furnaces which again require considerable cost in labor. The use of modules has also been found to be of value in relining old dense refractory linings such as refractory brick. In this case the folded or pleated modules are glued to the old monolithic or brick lining by use of a refractory mortar or glue which is generally of the hydraulic setting nature. This method of relining of kilns is labor intensive requiring the application of mortar to the brick work and then quickly applying the folded modules thereon. This method requires that time be given for the setting of the hydraulic mortar before the newly lined kiln can be fired.
All the above methods have one feature in common which is that they present an unbonded refractory fiber lining to the interior of the kiln. As time goes by these fibers devitrify (crystallize) and in so doing lose some of their mechanical strength making them more susceptible to mechanical abrasion and destruction due to high velocity gases from the heating mechanisms of the kilns.
High temperature resistant binder powders have been mixed with water and sprayed with refractory fibers. Such methods typically employ calcium alumina cements which are very alkaline and will settle when in suspension. It is also difficult to achieve homogenous mixing of fiber and binder with such a system. A hydraulic bond must be achieved prior to drying of the cement otherwise bonding of the fibers will not occur. In addition, careful drying of the sprayed mixture must be undertaken in order to prevent violent spalling.
Other methods for spraying insulation have been employed in the past utilizing organic binders. For example, U.S. Pat. No. 2,929,436 issued on Mar. 22, 1960 discloses a method and apparatus for spraying a mixture of fibers and resin material. As indicative of prior methods of application, this patent discloses mixing fiber and resin in a nozzle and spraying the mixture onto a surface. However, while suited for its intended purpose, such method and apparatus, if used in the spraying of refractory fiber with a tacky inorganic binder, would cause clogging of the nozzle. In addition, incomplete coating of the refractory fibers could cause a weakened layer of fibers which in turn could cause failure of the insulating layer at high temperatures. Because of the temperatures to which refractory fibers are subjected, often 2,000.degree. F. or more, such failures could be dangerous to workers in such industries as the steel industry and could cause extensive damage to furnaces and kilns. Thus, the intimate admixture of refractory fiber and the tacky inorganic binder to form a monolithic layer is important to the safe and efficient operation of such furnaces and kilns.